In resource exploration, drill strings are used for drilling the wellbore and assessing downhole conditions. A typical drill string includes a bit, followed by a series of inter-connected downhole tools. Directly above the bit, there are “at-bit” tools which can be connected together with direct electrical contacts. Above those tools is the mud motor which generally blocks direct electrical connections. Above the mud motor, there is a second set of inter-connected downhole tools. Because direct wire contacts are blocked by the motor, communication between the two sets of tools is often achieved using electromagnetic (EM) pulses on the outside of the drill string.
Above the downhole tools, there is a long series of pipe sections reaching to the surface and the drilling rig. Mud is pumped down this pipe at a high pressure and this mud turns the mud motor and flows through the bit to help cut the borehole and clear the cuttings at the borehole.
Many downhole tools are used to acquire data about the environment downhole (e.g., information about the formation) and this data needs to be transmitted to the surface. The information can be used in different ways. For instance, a human operator may use this data to determine what needs to be done to steer the entire drill string towards the desired target.
The downhole tools are generally powered with some combination of batteries and generators which turn the flow of mud into electrical power.
There are generally five kinds of pulses that are used to transmit data from downhole to the surface: mud pulses, EM pulses, sonic pulses, and direct wire connections. Mud pulses periodically restrict the flow of drilling fluid which causes pressure variations which can be detected on the surface. EM pulses use low frequency radio waves to send information upwards. Sonic pulses send sound waves into the metal body of the pipe to be detected on the surface. Direct wire connections are not used during drilling because they would be destroyed and all of the other communication mediums are very slow (i.e., measured in bits per second), and so there is a strong desire to transmit more data to the surface.